1. Field of the Invention
The present invention relates to a reception method for decoding, using a soft-decision value, a recovered bit stream that was encoded for transmission using an error correction code and further relates to a reception apparatus and a program using the reception method.
2. Description of the Background Art
As is conventionally known, a digital wireless communication system such as a wireless local area network (LAN) transmits and receives packet data between multiple communication devices according to wireless communication once network access is established. One contention-based access control method used for accessing a digital wireless communication system is the so-called carrier sense multiple access (CSMA) method.
When transmitting data based on a CSMA method, a communication device detects or senses the presence of a carrier on a corresponding channel used for communication with a destination communication device. When the channel is idle, that is, when no carrier is detected, the device transmits data. When the channel is busy, that is, when a carrier is detected, the device waits and, when the channel becomes idle, the data is then transmitted. In the above-described manner, the CSMA method provides method of control so as to prevent collisions, that is, interference between the transmitted signal and a signal transmitted from other communication devices.
However, even when providing access control such as by using a CSMA method, a digital wireless communication system cannot completely avoid interference because of the so-called hidden terminal problem, which is associated with a condition that is incapable of being detected through carrier sense methods as described.
Let us suppose that terminals B and C are situated within a communication range associated with a terminal A, and are outside of each other's communication range. In the present example, terminals B and C cannot detect each other despite performing carrier sensing. While terminal B is transmitting data to terminal A, terminal C may also transmit data to terminal A creating interference between the signals to be received by terminal A from both terminal B and terminal C. Thus to address the hidden terminal problem other method must be used, such as coding or the like.
Convolution coding is a known coding method used for improving the performance of digital wireless communication system and the accuracy of signal recovery despite the presence or occurrence of interference. Viterbi decoding is widely used as a decoding method featuring an enhanced error correction capability.
Viterbi decoding estimates a state transition of an encoder based on data extracted from a received signal. Based on a priori knowledge of the possible code states, the most likely or maximum likelihood state transition is established in order to estimate the actual code value of the received data extracted from a transmitted signal. A Hamming distance between a received code and a candidate code is generally used for quantitatively representing likelihood of the prospective state transition. Hard and soft decisions are known for finding the likelihood. The hard decision represents a received signal using 0 and 1 in binary. The soft decision represents a received signal using multiple values corresponding to amplitudes of the received signal.
A decision value generated from the hard decision has the same effect on decoding regardless of the reliability associated with the decision value itself. In contrast, a decision value generated from a soft decision has a little effect on decoding when the decision value itself is determined to be less reliable. Accordingly, use of soft decisions in decoding ensures higher reliability than the use of hard decisions. The following description uses the term “soft-decision value” to signify a value or a decision value used in soft decision making to represent the likelihood.
As described in Japanese Unexamined Publication No. 2001-257604 A (“JP-A-2001-257604”), for example, the proposed control method uses characteristics of the soft decision during decoding. When a soft-decision value is generated during the occurrence of interference, the value is replaced by a value representing a zero likelihood. In the above described manner, the proposed method prevents the soft-decision value obtained during interference from contributing to the Viterbi decoding result.
The conventional apparatus described in JP-A-2001-257604 assumes the presence of a high-power interference wave periodically transmitted from another system, such as a weather radar, and uses interference wave characteristics to detect an interference occurrence period. The apparatus requires an additional special detection circuit for detecting an interference wave and is thus relatively large and expensive.